Incremental optical motion encoders are used to resolve the position and movement of an object along a particular route. Such encoders generally include a light source for emitting a light beam, light modulation means for modulating the light beam in response to movement of the object along the route, and detection means for receiving modulated light and for producing discrete electrical signals representing detection of light by the detectors. As the light is modulated in response to the movement of the object, the stream of electrical signals from the detector assembly produces a continuous wave form usually resembling a square wave. The position of the object along its route determines the position of each signal in the wave form. The phase of the wave form differs depending upon the location of the object. Thus, signals from the detectors can be used to indicate a change in location of the object along the route. Two or more out-of-phase signals from separate detectors can be used to detect both change in location and change of direction of movement.
For an incremental motion encoder to indicate the absolute position or location of the object along its route, an index pulse is generated at least once along the route. The incremental signals can be used to count incremental movement from the index pulse. If the position of the object is known at the time the index pulse is generated, the absolute position of the object at any place along the route can be determined.
Therefore, to provide an indication of absolute position, change in location and direction of movement, an incremental encoder usually requires three channels of information. Two channels are derived from two or more out of phase encoder signals that are produced throughout the route of the object, and a third is an index signal produced at least once along the route at a known position of the object.
Such a position encoder or a movement detector is used to measure the angular position of a shaft. Depending on the use of such a shaft angle encoder, a high degree of resolution and accuracy may be needed; for example, an automotive crankshaft angle measurement applications or accelerator speed control, a resolution of 2000 increments per revolution of the shaft may be necessary. Accuracy of the correlation between the signal from the encoder and the actual mechanical position of the shaft is also important. Mechanical alignment discrepancies can adversely affect accuracy as can electrical noise, due to the very small dimensions of the code wheel, the transmissive sections and the nontransmissive sections.
To accurately detect the index pulse, a push-pull electronic arrangement may be used to determine the location of the index pulse. In such an arrangement, two photodetectors are arranged laterally adjacent one another in alignment with a light source; a circular code wheel fixed to the shaft separates the light source and the detectors. When one of the detectors is illuminated, a logic signal of one sense, such as a "1", is produced. When a spoke on the code wheel occults the illumination, the opposite logic signal is produced, such as a logical "0." Then as the index window passes over the detectors, one detector will generate a logic 1 of long duration, followed an instant later by a logic 1 from the second detector. The direction of travel of the code wheel may be determined by sensing which detector is first to generate a long duration of logic 1.
An optical encoder that produces an index pulse having the same logic value regardless of the direction of rotation of the code wheel has three photodetectors arranged laterally adjacent one another in alignment with the light source. The principal photodetector has a unit width; the two secondary photodetectors each have a one-half unit width. The secondary photodetectors are preferably arranged to be occulted by a spoke or adjacent spokes of the code wheel when the principal photodetector is illuminated by the light source. The principal photodetector is preferably mounted between the second and third photodetectors. Alternatively, the two secondary photodetectors can be mounted on one side of the principal photodetector. Each photodetector can produce a logic level in response to the detection of light or the absence of light.
The code member has a code wheel with a circumferential track comprising a plurality of alternating windows and spokes for alternately transmitting or blocking the light upon movement of the code wheel. Each window and each spoke has a unit width. The encoder preferably includes a circuit for processing the photodetector output signals. The circuit comprises a buffer circuit for processing the output signals, having a first input connected to the output of the two secondary photodetectors, and a second input connected to the output of the principal photodetector.
A desirable feature of the optical encoder is the capability of using code wheels of differing density with a particular photodetector. The density of a code wheel is defined by the size of the principal photodetector relative to the size of the window or spoke of the code wheel. For example, a code wheel of equal density to that of the photodetector has a window and spoke each with a width equal to the width of the principal photodetector. Similarly a half density code wheel has a window and spoke each with a width that is twice the width of the principal photodetector.
A half density code wheel also functions as a slot interrupter because a window allows the light source to totally illuminate both the principal and secondary photodetectors, and a spoke completely occults both the principal and secondary photodetectors. However, for the previously described embodiment, the photodetector can produce an ambiguous output signal when both the principal and secondary photodetectors are either entirely illuminated or entirely occulted.
Thus it is desirable to provide an optical encoder apparatus which produces an unambiguous output signal for code wheels having equal or lesser density than that of the photodetectors. Preferably this is provided by modifying the photodetector rather than changing external hardware.